The Pope has declared Mother Teresa a saint.

Tens of thousands watched in St Peter's Square in the Vatican as Pope Francis led the canonisation mass for the nun.

The pontiff said in Latin: "We declare and define Blessed Teresa of Calcutta (Kolkata) to be a Saint and we enroll her among the Saints, decreeing that she is to be venerated as such by the whole Church."

Hundreds of sisters from Mother Teresa's Missionaries of Charity - recgonisable by their blue-trimmed saris (garment) - attended the ceremony.

Image: Gregorio Borgia / AP/Press Association Images

Her elevation to sainthood comes on the eve of the 19th anniversary of her death.

The nun - a Nobel Peace Prize winner - devoted her life to helping the poor in the slums of the Indian city of Calcutta.

Ahead of today's ceremony, the pontiff said the charity worker "made works of mercy the guide of her life".

Let us imitate Mother Teresa who made works of mercy the guide of her life and the path towards holiness.

— Pope Francis (@Pontifex) September 3, 2016

The sainthood process for Mother Teresa was commenced by the late Pope John Paul II in 2003, when she was beatified.

To become a saint, the church has to verify two "miracles".

Catholic authorities have recognised the claims of two people who said they survived incurable tumours thanks to Mother Teresa.

However, Mother Teresa has also been the subject of controversy over the years.

Her foremost critic was the late writer Christopher Hitchens, who published a book and made a documentary questioning her beliefs and approach.

In an essay, he wrote: "MT was not a friend of the poor. She was a friend of poverty. She said that suffering was a gift from God.

"She spent her life opposing the only known cure for poverty, which is the empowerment of women and the emancipation of them from a livestock version of compulsory reproduction."

Academics and researchers also questioned the quality of care offered at her facilities in India.